Natural killer (NK) cells participate in early immune responses to many microbial pathogens, but are particularly important in host defense against herpesvirus infections. NK cells also reject a variety of tumors in vitro and in vivo. NK cells manifest potent cytotoxicity against altered (infected or transformed) cells, and through interactions with dendritic cells and the secretion of cytokines (e.g. IFN-gamma), NK cells influence the subsequent adaptive immune response. The research program's long-term goal is to understand how NK cells participate in coordinated immune responses and in disease. To reduce the inherent complexity of NK cell biology, a powerful in vitro reporter system has been generated to examine the activation of isolated NK cell receptors, which led to the identification of a murine cytomegalovirus (MCMV)- encoded ligand for the activating NK cell receptor Ly-49H. This proposal seeks to dissect the accessory molecular interactions that contribute to MCMV-mediated activation of Ly-49H through Specific Aims: 1) define the host cell lineage restrictions (specific aim la) and requirements for host cell expression of MHC class I and related structures (specific aim lb) that contribute to Ly-49H-mediated activation; 2) a test for costimulatory input to primary Ly49H-MCMV activation mediated by NKG2D, 2B4, and/or 4-1BB (CDw137); and 3) a test whether stimulatory members of the human killer cell immunoqlobulin-like receptors (KIR) are specifically activated by host cells infected with human herpesviruses (HCMV, EBV, and HSV). Results from Ly-49 receptor reporter cells are measured qualitatively and quantitatively, and will be verified in mouse spleen-derived Ly-49H+ NK cell subsets by examination of proliferation, cytokine production, and cytotoxicity. The interactions that regulate Ly-49H-mediated activation in vivo will be assessed by MCMV infection of selected strains of C57BL/6 mice carrying informative mutations (e.g., in MHC class I and related loci). The results will advance our insight on the cellular and molecular bridges between innate and adaptive immunity.